The developing apparatus includes a cylindrical magnet roller opposing a photoconductive drum, a developing sleeve having a peripheral surface which carries developer and being driven to rotate around the magnet roller, and a doctor blade configured to regulate the thickness of a layer of the developer on the peripheral surface of the developing sleeve.
The magnet roller has a plurality of magnetic poles in the interior thereof. The action of a magnetic force forms a magnetic brush formed of magnetic carriers having non-magnetic toner adsorbed thereon on the peripheral surface of the developing sleeve.
The magnetic blush supplies toner to an electrostatic latent image by the magnet roller and the photoconductive drum rotating in a state in which the magnetic blush is in contact with the peripheral surface of the photoconductive drum.
The peripheral surface of the developing sleeve is machined into a rough surface in order to carry the developer of an adequate amount on the peripheral surface thereof and supply the toner to the photoconductive drum, and in order to prevent the developer from slipping on the peripheral surface of the sleeve.
As an example of machining, sandblasting and knurling are known.
In the sandblasting, particles of regular shape or irregular shape are used as abrasives and minute concave-convex are formed on the peripheral surface of the sleeve. In the knurling, a groove extending in parallel with the axis of rotation of the magnet roller is formed over the entire circumference of the peripheral surface of the sleeve.
In the related art, a developing apparatus which employs the developing sleeve having a V-groove is known (U.S. Pat. No. 6,925,277).
The developing apparatus disclosed in U.S. Pat. No. 6,925,277 has a configuration in which the surface of the developing sleeve in a central portion including an image-creating area which corresponds to an image-forming area on a photoconductor is provided with a developer transporting capability higher than the surfaces at the both end portions of the developing sleeve positioned outside the central portion in the widthwise direction being orthogonal to the direction of movement of the surface of the developing sleeve.
In the developing apparatus, the both ends of the magnetic pole of the magnet roller in the widthwise direction are opposed to the surfaces of the developing sleeve at both ends thereof.
In JP-A-2008-139650, a method of uniformizing the amount of transport of the two-component developer in the axial direction after the passage through a layer thickness regulating member by making the capability of the surface of the developing sleeve to carry and transport the two-component developer weaker in the axially central portion than in the axially both end portions is disclosed.
The concave-convex on the peripheral surface of the sandblasted developing sleeve is flattened by the developer during a long time usage. Consequently, the frictional resistance between the peripheral surface of the sleeve and the carrier is lowered and hence the developing sleeve cannot transport the developer stably.
In contrast, the grooves formed by knurling are deeper than the recesses formed by the sandblasting, and hence lowering of the frictional resistance caused by the long time usage is not significantly large.
However, if the knurled developing sleeve is used for developing, a needless pattern of lines having a width corresponding to the groove pitch is disadvantageously generated on the developed toner image.
If the sandblasted developing sleeve is used for developing, the needless pattern of lines is not generated. However, the stability of transport of the developer is disadvantageously impaired by deterioration with age.